This probably won't be up for long. Not at this price. But the Annotated Sandman Vol. 3 is currently listed on Amazon for $19.71, compared to the $49.99 cover price.
That's 61% off.
Published in October 2014, this is far cheaper than even comic stores and bookstores will be able to buy the volume from DC and Diamond Comic Distrubutors for.
Which means, while it is at this price, comic stores and bookstores might want to load up themselves, before the inevitable price changes.
After all, Sandman Overture will still be coming out then…